1. Field of the Invention
The present invention relates to a measurement system and, more particularly, to a method of detecting measurement errors for critical dimension (CD) values of a pattern in a measurement system with a scanning electron microscope.
2. Description of the Related Art
Typically, there are two methods to measure critical dimension (CD) values: (1) a manual CD measuring method; and (2) an automatic CD measuring method. In the manual CD measuring method, an operator manually performs a CD measurement. Obtaining an accurate CD measurement with the manual CD measuring method requires a highly experienced operator. Therefore, the manual CD measuring method has problems in that the reliability of measured data depends on the skill of the operator. The automatic CD measuring method automatically detects an edge of a fine pattern using an image processing means and determines a CD of the pattern by data located at the edge.
In addition, as semiconductor devices become more highly integrated, demand for measurement of small-sized patterns increases, and a large amount of measured data is required for process stability. In order to obtain such data, measurement systems (or apparatuses) are automated.
FIG. 1 illustrates a conventional automatic measurement system. Such an automatic measurement system of FIG. 1 may include a computer 10 and a scanning electron microscope 20. The computer 10 may include a monitor and a random access memory (RAM), wherein a software module having a function for measuring a critical dimension (CD) is stored.
In order to measure a CD value of a fine pattern, such as that formed on a semiconductor wafer, first the software in the computer 10 is executed to display a graphic user interface (GUI) on the monitor. The computer 10 controls the operation of the scanning electron microscope 20 to scan electron beams over the fine pattern. The computer 10 detects secondary electrons generated from the scanned portion of the pattern to create and display an image of the pattern on the monitor. Thereafter, a CD value (i.e., width and length) of the fine pattern may be measured.
FIG. 2 illustrates a cross-sectional view of a semiconductor wafer showing a reflection/radiation pattern formed by a semiconductor wafer. In order to determine different image contrasts contained in a pattern 32, the electron beam is scanned over a flat portion 32a, a tapered portion 32b and an edge portion 32c of the pattern 32. When the electron beams 30 are scanned over flat portion 32a of a pattern 32 formed on a semiconductor wafer 34, relatively few secondary electrons “e−” are emitted. When the electron beams 30 are scanned over tapered portion 32b of the pattern 32, more secondary electrons “e−” are radiated at the tapered portion 32b than at the flat portion 32a. Further, when the electron beams 30 are scanned over an edge portion 32c of the pattern 32, more secondary electrons “e−” are radiated at the edge portion 32c than at the tapered portion 32b. 
Brightness of an image of the pattern displayed on a monitor is proportional to the number of the radiated secondary electrons e−. In other words, a portion of the pattern that radiates a high number of secondary electrons e− is displayed on a monitor of the computer with a relatively high brightness. Alternatively, a portion of the pattern that radiates fewer secondary electrons e− is displayed on the monitor of the computer with a relatively low brightness. Therefore, the pattern image displayed on the monitor using contrasts represents the pattern over which the electron beam is scanned.
FIG. 3 illustrates an image of the pattern displayed on a monitor. As described above, the pattern 32 is displayed as an image on the monitor 36. Thereafter, a CD value of the pattern 32 can be measured.
In such a conventional automatic measurement system, it is very important to have reliable resulting data (i.e., CD value) measurements. For example, when measured data has measurement errors, the reliability of the measured data is reduced, and data that are not measured may be higher in reliability than the measured data. In addition, the conventional automatic measurement system merely measures the CD value of the image and then displays the CD value on a monitor, and thus there is no method for determining whether the measured CD value has errors. Therefore, reliability for the measured CD value is reduced.